[PATCH] user-cr: invoke exit system call directly from ckpt_do_feeder

[Oren Laadan]

Nice catch, Queued for v19-rc2.

Nathan Lynch wrote:
> The feeder thread can cause the restart process to fail by indirectly
> calling exit_group, which sends SIGKILL to all other threads in the
> process.  If the feeder thread "wins" the race, the restart is
> disrupted.  A common symptom of this race is the coordinator task
> returning from the wait_for_completion_interruptible in
> wait_all_tasks_finish with a signal (the SIGKILL) pending.
> 
> Calling _exit isn't enough; see
> http://www.kernel.org/doc/man-pages/online/pages/man2/exit.2.html#NOTES
> 
> Exit the feeder thread by using the syscall() macro.
> 
> Signed-off-by: Nathan Lynch <ntl at pobox.com>
> ---
>  restart.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/restart.c b/restart.c
> index d5d069a..ed4268c 100644
> --- a/restart.c
> +++ b/restart.c
> @@ -2079,8 +2079,16 @@ static int ckpt_do_feeder(void *data)
>  		ckpt_read_write_inspect(ctx);
>  	else
>  		ckpt_read_write_blind(ctx);
> -		
> -	/* all is well: feeder thread is done */
> +
> +	/* All is well: feeder thread is done.  However, we must
> +	 * invoke the exit system call directly. Otherwise, upon
> +	 * return from this function, glibc's clone wrapper will call
> +	 * _exit, which calls exit_group, which will terminate the
> +	 * whole process, which is not what we want.
> +	 */
> +	syscall(SYS_exit, 0);
> +
> +	/* not reached */
>  	return 0;
>  }
>